1. Field of the Invention
This invention relates to a heterojunction field effect transistor, and it is especially suitable for use in a heterojunction field effect transistor using nitride-based III-V compound semiconductors such as GaN.
2. Description of the Related Art
Nitride-based III-V compound semiconductors including GaN as their major component (hereinafter called "GaN semiconductors") are direct transitional semiconductors having energy gaps (band gaps) E.sub.g ranging from 1.9 eV to 6.2 eV and theoretically enabling realization of light emitting devices capable of emitting light ranging from visible ranges to ultraviolet ranges. Therefore, semiconductor light emitting devices using GaN semiconductors are now being under active developments. GaN semiconductors also have a great possibility as materials of electron moving devices. That is, the saturation electron velocity of GaN is as large as about 2.times.10.sup.7 cm/s, which is larger than that of Si, GaAs or SiC, and its breakdown electric field is as large as 5.times.10.sup.6 V/cm, which is next to that of diamond. For these reasons, GaN semiconductors have been expected to be hopeful as materials of high-frequency, high-power semiconductor devices.
Because of the difficulty of making p-type layers with a high carrier concentration, no bipolar transistor as an electron moving device using GaN semiconductors has been made, and there are only field effect transistors (FET) on an experimental basis till now. FET using GaN semiconductors has three serious handicaps as compared with FET using GaAs semiconductors. The first one is the difficulty in making the conduction layer by ion implantation or diffusion of an impurity, the second is the difficulty of making an alloy with a metal, and the third is that etching relies on reactive ion etching (RIE) or its similar process, and no gate recess technology with a high accuracy has been developed. Therefore, developments of GaN FETs made heretofore have been within these restrictions.
Shown in FIG. 1 is a conventional AlGaN/GaN heterojunction FET (Appl. Phys. Lett. 65(9), 1121(1994)). As shown in FIG. 1, the AlGaN/GaN heterojunction FET has formed an n-type GaN channel layer 102 as an electron moving layer and an n-type Al.sub.0.13 Ga.sub.0.87 electron supply layer 103 on a sapphire substrate 101. The n-type GaN channel layer 102 is 0.6 .mu.m thick, and the n-type Al.sub.0.13 Ga.sub.0.87 N electron supply layer 103 is 25 nm thick. The n-type Al.sub.0.13 Ga.sub.0.87 N electron supply layer 103 is patterned in a predetermined configuration, probably by non-selective etching by RIE. A gate electrode 104 is formed on the n-type Al.sub.0.13 Ga.sub.0.87 N electron supply layer 103, and a source electrode 105 and a drain electrode 106 are formed on the n-type GaN channel layer 102 in contact with opposite side walls of the n-type Al.sub.0.13 Ga.sub.0.87 N electron supply layer 103. The gate electrode 104 is in Schottky contact with the n-type Al.sub.0.13 Ga.sub.0.87 N electron supply layer 103, and the source electrode 105 and the drain electrode 106 are in ohmic contact with the n-type GaN channel layer 102. Thus, the AlGaN/GaN heterojunction FET has a structure similar to a high electron mobility transistor (HEMT), but is different from normal HEMT in using the n-type GaN channel layer 102 in lieu of an intrinsic GaN layer as the electron moving layer.
In the conventional AlGaN/GaN heterojunction FET shown in FIG. 1, the n-type Al.sub.0.13 Ga.sub.0.87 N layer 103 must be patterned by non-selective etching to bring the source electrode 105 and the drain electrode 106 into direct contact with the n-type GaN channel layer 102 relatively good in ohmic contact. Therefore, the n-type GaN channel layer 103 must be as thick as 0.6 .mu.m, and this is a major reason of degradation of the performance of the FET itself.
FIG. 2 shows another conventional AlGaN/GaN heterojunction FET (Appl. Phys. Lett. 69(6), 794(1996)). As shown in FIG. 2, the AlGaN/GaN heterojunction FET includes an intrinsic GaN layer 202, n-type GaN channel layer 203 as an electron moving layer, undoped Al.sub.0.15 Ga.sub.0.85 N spacer layer 204, and n-type Al.sub.0.15 Ga.sub.0.85 N electron supply layer 205 sequentially stacked on a sapphire substrate 201. The intrinsic GaN layer 202 is 1 .mu.m thick, n-type GaN channel layer 203 is 0.1 .mu.m thick, undoped Al.sub.0.15 Ga.sub.0.85 N spacer layer 204 is 3 nm thick, and n-type Al.sub.0.15 Ga.sub.0.855 N electron supply layer 205 is 30 nm thick. Formed on the n-type Al.sub.0.15 Ga.sub.0.85 N electron supply layer 205 are a gate electrode 206, source electrode 207 and drain electrode 208. The gate electrode 206 is in Schottky contact with the n-type Al.sub.0.15 Ga.sub.0.85 N electron supply layer 205, and the source electrode 207 and the drain electrode 208 are in ohmic contact with the n-type Al.sub.0.15 Ga.sub.0.85 N electron supply layer 205.
Also in the conventional AlGaN/GaN heterojunction FET shown in FIG. 2, the thickness of the n-type GaN channel layer 203 as thick as 0.1 .mu.m causes degradation of the performance of the FET itself.
FIG. 3 shows a conventional AlGaN/GaN HEMT having a thinner electron moving layer for a higher performance (Appl. Phys. Lett. 68(20), 2849(1996)). As shown in FIG. 3, the AlGaN/GaN HEMT includes an AlN buffer layer 302, undoped GaN layer 303, undoped Al.sub.0.16 Ga.sub.0.84 N layer 304, undoped GaN channel layer 305 as an electron moving layer, undoped Al.sub.0.16 Ga.sub.0.84 N spacer layer 306, n-type Al.sub.0.16 Ga.sub.0.84 N electron supply layer 307, undoped Al.sub.0.16 Ga.sub.0.84 N barrier layer 308 and n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309 sequentially stacked on a sapphire substrate 301. The undoped GaN layer 303 is 3 to 5 .mu.m thick, undoped Al.sub.0.16 Ga.sub.0.84 N layer 304 is 20 nm thick, undoped GaN channel layer 305 is 7.5 nm thick, undoped Al.sub.0.16 Ga.sub.0.84 N spacer layer 306 is 5 nm thick, n-type Al.sub.0.16 Ga.sub.0.84 N electron supply layer 307 is 2 nm thick, undoped Al.sub.0.16 Ga.sub.0.4 N barrier layer 308 is 13 nm thick, and n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309 is 6 nm thick. Formed on the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309 are a gate electrode 310, source electrode 311 and drain electrode 312. The gate electrode 310 is in Schottky contact with the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309, and the source electrode 311 and the drain electrode 312 are in ohmic contact with the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309. The gate electrode 310 is made of a Ti/Pd/Au film, and the source electrode 311 and the drain electrode 312 are made of Ni/AuSi/Ag/Au films.
In the conventional AlGaN/GaN HEMT shown in FIG. 3, the thickness of the undoped GaN channel layer 305 as the electron moving layer is as thin as 7.5 nm. However, since the source electrode 311 and the drain electrode 312 are formed on the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309, good ohmic contact is not obtained. Therefore, the source resistance is high, and the essential performance of AlGaN/GaN HEMT could not be obtained. In general, the transconductance g.sub.m, which is one of indices of the performance of FET, is g.sub.m =g.sub.mi /(1+R.sub.s g.sub.mi), where R.sub.s is the source resistance, and g.sub.mi is the intrinsic transconductance (transconductance when R.sub.s =0) This equation shows that, with a large R.sub.s, g.sub.m becomes small as compared with g.sub.mi, and an acceptably large current driving power cannot be obtained.
To improve the ohmic contact of the source electrode 311 and the drain electrode 312, it would be a possible approach to alloy the source electrode 311 and the drain electrode 312 with the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer 309. However, AlGaN is very hard, and has a high melting point (the melting point of AlN is 3000.degree. C., and the melting point of GaN is higher than 1700.degree. C.). Therefore, AlGaN is not solid-soluble with metals, and it is extremely difficult to obtain a low-resistance ohmic contact by alloying.